1. Field of the Invention
The present invention relates to a transfer roller.
2. Description of the Related Art
With conventional electrophotographic printers, a charging device uniformly charges the surface of a photoconductive drum and then an LED head illuminates the charged surface to form an electrostatic latent image. The electrostatic latent is developed by a developing device into a toner image. Then, the toner image is transferred by a transfer device to a print medium. A fixing device subsequently fixes the toner image on the print medium.
FIG. 7 is a perspective view of a transfer device of a conventional electrophotographic printer.
A transfer roller 11 is in pressure contact with a photoconductive drum 12. The transfer roller 11 rotates in a direction shown by arrow A and the photoconductive drum 12 rotates in a direction shown by arrow B. The transfer roller 11 receives a transfer voltage or positive high voltage +E. The photoconductive drum 12 carries a toner image 13 formed thereon. When a print medium 14 passes between the transfer roller 11 and the photoconductive drum 12, a transfer current flows from the transfer roller 11 to the photoconductive drum 12, thereby causing the toner image 13 to be deposited on the print medium 14.
The transfer roller 11 has a uniform surface resistance, so that a uniform transfer current flows through any areas in contact with the photoconductive drum 12. The transfer roller 11 is urged by a constant pressure F against the photoconductive drum 12, thereby creating an appropriate longitudinal contact area between the transfer roller 11 and the photoconductive drum 12. This ensures that the print medium 14 travels straightforwardly in a direction shown by arrow C as the transfer roller 11 and the photoconductive drum 12 rotate.
The transfer roller 11 includes a metal shaft with a rubber material deposited thereon. The rubber material is formed by kneading, for example, a semiconductive urethane rubber, silicone rubber, or the like. The rubber material is foamed, vulcanized into a sponge-like material by adding a vulcanizing agent, and finally formed into a specific shape and dimensions. An electrically conductive material is added to the rubber material, thereby making the rubber material electrically semiconductive.
The conventional transfer roller 11 suffers from the following problems. Some rubber materials react with the material of the photoconductive drum 12 when they are left in an environment of high temperature and high humidity for a long time, so that the rubber material welds with the photoconductive drum, or the vulcanizing agent, semiconductive material, or the like is deposited to the photoconductive drum. As a result, the photoconductive drum is contaminated, causing lines to appear in the printed image. This decreases the print quality.
In order to solve this problem, the length of time during which vulcanization is performed is extended so that less vulcanizing agent is added to the rubber material, or the surface of the transfer roller 11 is coated with an inert material. However, such treatments not only are insufficient in preventing drum contamination but also lose some degrees of freedom in designing the characteristics of transfer roller 11 such as electrical resistance and mechanical hardness.